A small town in Finland is experimenting with a new type of infrastructure: the world's largest sand battery. The battery-a 42-foot-tall, nearly 50-foot-wide silo filled with 2,000 tons of crushed stone-sits on the edge of a parking lot. When there's extra renewable electricity on the grid and power is cheap, the system uses electricity to heat up the crushed stone. That heat is stored in the battery until nearby buildings need to use it.